We are presently characterizing the structure of E. Coli RNA polymerase ternary complexes on SV40 DNA so as to better understand the mechanism of pausing and termination. We have successfully determined the unwinding angles at both 5 degree and 37 degree of binary, initiation, and ternary complexes of the polymerase with DNA. From this data, which indicates a DNA bubble of 18 plus or minus 2 base pairs in the ternary complex, we have constructed a model of the transcriptional complex. This model invokes the existence of two swivelase activities which maintain the integrity of the bubble as the polymerase moves down the helix. Our model suggests that a DNA-RNA bridged structure at the distal end of the bubble may play a role in pausing and termination. We intend to refine our model using DNA sequencing and computer modeling technology and to extend our experiments to eucaryotic RNA polymerases and to ternary complexes blocked at unique sites on the genome by a psoralen monoadduct or crosslink. Towards this goal we are developing an enzymatic route to the site specific incorporation of psoralen modified dTTP.